The present invention relates to the calibration of optical instruments and more particularly to apparatus for calibrating a densitometer or the like.
The use of instrumentation such as the densitometer for measuring optical density is becoming more commonplace in many fields. For example, the assignee of this application has recently developed a dosimeter for use in monitoring the treatment of hyperbilirubinemia in infants by phototherapeutic means. The optical density (a measurement of the ability of a material to transmit light) of the dosimeter changes in response to the equivalent destruction of serum bilirubin in the bloodstream of the infant by light irradiation. Determination of the amount of bilirubin decomposed is dependent upon accurate measurement of the change in optical density of the dosimeter. Numerous federal and state agencies are requiring that means for certifying performance criteria be provided for instrumentation used in such medical diagnoses.
Many materials have the ability to transmit or absorb certain frequencies of light. In the fields of compositional analysis, of which clinical medicine is an important part, this phenomenon has been put to good use. If white light containing the full spectrum of light frequencies is passed through a material sample, the light emerging from the sample will be modified according to the absorptive qualities of the material as to each of the frequencies of the light. Such "filtered" light then can be analyzed, and by comparison to known standards, the composition of the material or presence of specific components can be determined. In a typical scanning densitometer performing the above analysis, the sample, deposited as a thin film on a transparent substrate, is moved along a scan axis and across a zone of illumination in the densitometer. The light passing through the sample is detected and converted to an output signal which may be integrated to eliminate the effects of localized anomalies in the sample film. In this regard, the densitometer generates a chart trace of the output signal during the scanning operation together with a listing of integrated values of the output signal equivalent to areas under the chart trace. Such information enables a skilled technician to draw conclusions concerning the nature of the scanned sample.
Of course, it is important that such densitometers be accurately calibrated and that such calibration be maintained during the operation thereof. Typically, the calibration and testing process comprises causing the densitometer to scan a standard presenting a known optical density equivalent and area. The response of a correctly calibrated and operating densitometer when scanning the standard can be anticipated. By comparing the output of the densitometer scanning the standard to the expected output, the performance of the subject densitometer can be determined and instrument adjustment made to bring the densitometer performance into correlation with the standard.
In the past, such densitometer calibrating standards have been limited by the filter employed therein. In this regard, various filter types are available for incorporation into a standard. However, each has limitations in one or more areas of accuracy, linearity and cost -- particularly when it is desired to mass produce calibration standards for use with a widely distributed product line of densitometers or the like.
Ideally, a filter for use in a standard should be accurate. That is, it should present an actual optical density equivalent equal to the intended optical density equivalent. Further, the filter should be producible at low cost in large quantities with uniform filter characteristics. Also the filter should be linear with wavelength. That is, it presents the same optical density equivalent regardless of the wavelength of the light passing therethrough.